SELECT 
  cscart_product_descriptions.product_id, 
  cscart_product_descriptions.unit_name 
FROM 
  cscart_product_descriptions 
WHERE 
  cscart_product_descriptions.product_id IN (
    20973, 20974, 20975, 20976, 20977, 20978, 
    20979, 20980, 20981, 20982, 20983, 
    20984, 20985, 20986, 20987, 20988
  ) 
  AND cscart_product_descriptions.lang_code = 'en'

Query time 0.00916

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_descriptions",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "product_id", "idx_product_lang"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["product_id", "lang_code"],
      "rows": 16,
      "filtered": 100,
      "index_condition": "cscart_product_descriptions.product_id in (20973,20974,20975,20976,20977,20978,20979,20980,20981,20982,20983,20984,20985,20986,20987,20988) and cscart_product_descriptions.lang_code = 'en'"
    }
  }
}